Multi-spectral imaging (MSI) and hyper-spectral imaging (HSI) systems, in which more than 3 spectral bands are captured by an imager, are of particular interest for a variety of applications. Originally exploited for use in aerial and satellite imaging, spectral imaging apparatus and techniques are more recently being applied to a broad range of applications where the spectral content of light yields significant information about a structure or tissue, including remote sensing, industrial sensing, and biological and health imaging. As is described in U.S. Pat. No. 5,784,162 entitled “Spectral Bio-Imaging Methods for Biological Research, Medical Diagnostics, and Therapy” to Cabib et al., spectral imaging can be used to detect cell constituents such as proteins tagged with fluorescent probe substances, to distinguish cancer cells from healthy cells, and in a range of imaging applications. Spectral imaging shows significant promise, for example, in detection of precursor cell structures in cancer diagnosis. Unlike biopsy, which is invasive, offers delayed results, and can be highly localized so that it can miss some types of significant changes, spectral imaging techniques for cancer detection are minimally invasive, operate in real time, and can perform over sizable tissue areas. Spectral imaging techniques can be applied for detection and quantitative measurement of microscopic morphological and biochemical changes and are anticipated to serve as valuable tools for early disease detection. Spectral imaging has also been shown to be of value for wound assessment. Other medical applications served by spectral imaging methods include microscopy, endoscopy, and fundus imaging.
Other uses for spectral imaging include applications in industrial sensing and process control. Spectral imaging is advantaged over other imaging techniques since it is able to obtain a continuous spectral “fingerprint” for an image over a range of wavelengths including UV and IR. Because of this, spectral imaging is able to reveal the “hidden” morphology of an object under test. This enables spectral imaging systems to be used in applications such as non-contact detection systems for ascertaining fruit ripeness, for example. Still other uses for spectral imaging include remote sensing applications.
Multi-spectral imaging systems are typically designed with thin-film spectral filters situated in front of detector arrays. MSI systems provide good image quality with short detector integration times and modest sized data sets. They are used in both line-scanned configurations, with linear detector arrays, and in full-field configurations, with area detector arrays. The MSI approach can image only a relatively small number of spectral bands, typically from about 4 to about 8, with the band selection filter response fixed once the system is built.
Hyper-spectral imaging systems, on the other hand, use a dispersive element, such as a grating or prism, for example, to spatially disperse an input image signal onto an area detector array, providing one spatial axis and one spectral axis. HSI systems can be more capable than MSI systems, are able to capture a vast amount of spectral information in a number of very narrow spectral bands, and are generally more flexible than MSI systems. However, this added capability comes at a price: for example, signal acquisition times for HSI are much longer than for MSI, the data sets are extremely large and spatial resolution may be compromised. In practice, typically only a small subset of the captured hyper-spectral data cube is of interest.
There have been a number of proposed solutions for programmable spectrometers and programmable spectral imaging systems, in which the spectral transmission function can be more readily modified. Electronically controlled light modulators have been used in either of two fundamentally different base configurations:
1) as a type of tunable transmission filter; or,                2) as a programmable dispersion-based system, using a spatial light modulator consisting of an array of individually addressable devices.        
Tunable transmission filters have been disclosed using liquid crystal devices (LCDs), acousto-optical (AO) devices, and tunable Fabry-Perot cavities, for example. Liquid crystal tunable filters, which may be alternately called LC or LCD filters, have been disclosed in U.S. Pat. No. 5,689,317 entitled “TUNABLE COLOR FILTER” to Miller et al. issued Nov. 18, 1997 and in U.S. Pat. No. 5,892,612 entitled “TUNABLE OPTICAL FILTER WITH WHITE STATE” also to Miller et al., issued Apr. 6, 1999. An imaging apparatus using tunable LCD filters is disclosed in U.S. Pat. No. 6,760,475 entitled “COLORIMETRIC IMAGING SYSTEM” to Miller, issued Jul. 6, 2004.
While tunable LCD filters provide an effective solution for some imaging applications, these devices have some significant limitations. These limitations include some constraints on spectral range, temperature sensitivity, polarization sensitivity, relatively poor transmission characteristics, and relatively slow response times.
A programmable spectral imaging system using an acousto-optical tunable filter is disclosed, for example, in U.S. Pat. No. 5,828,451 entitled “SPECTRAL IMAGING SYSTEM AND METHOD EMPLOYING AN ACOUSTO-OPTIC TUNABLE FILTER FOR WAVELENGTH SELECTION WITH INCREASED FIELD OF VIEW BRIGHTNESS” to Bellus et al., issued Oct. 27, 1998. While tunable AO filters have utility some imaging applications, these devices have some significant limitations. These limitations include some spectral range constraints, relatively poor transmission characteristics, some reported temperature sensitivity, and constrained active area dimensions, with significant power requirements even for active areas of modest size.
A tunable Fabry-Perot filter for selecting visible wavelengths is disclosed in U.S. Pat. No. 6,295,130 entitled “STRUCTURE AND METHOD FOR A MICROELECTROMECHANICALLY TUNABLE FABRY-PEROT CAVITY SPECTROPHOTOMETER” to Sun et al., issued Sep. 25, 2001.
Programmable dispersion-based systems have been demonstrated using a variety of spatial light modulators, including liquid crystal display panels and, more recently, micro-electromechanical mirror arrays, such as the Digital Micromirror Device (DMD) used in Digital Light Processor components from Texas Instruments, Dallas, Tex. for example. In the dispersion-based approach, input light is dispersed through a prism or grating in order to separate the various component wavelengths onto the spatial light modulator. The spatial light modulator then selects the wavelengths of interest and directs these to a detector. Typically, dispersion based systems have used a single detector element, for point-imaging or non-imaging applications. In a single-detector configuration, a 2D image can be generated by raster scanning an object of interest by using some scanning mechanism, such as a pair of scanning mirrors, for example. A non-imaging DMD-based spectrometer for sample analysis is described by R. A. DeVerse et al. in “Realization of the Hadamard Multiplex Advantage Using a Programmable Optical Mask in a Dispersive Flat-Field Near-Infrared Spectrometer” Applied Spectroscopy. 54, pp. 1751-1758 (2000). This disclosure indicates that, when many narrow spectral bands are of interest, marked signal-to-noise improvement can be obtained by a method of simultaneously measuring multiple bands and applying a Hadamard transform approach, rather than by measuring the spectral bands sequentially.
A dispersion-based programmable spectral imager that uses a detector array rather than a single detector is described by C. M. Wehlburg et al. in “Optimization and Characterization of an Imaging Hadamard Spectrometer” Proc. SPIE 4381, pp. 506-515 (2001). The disclosed Hadamard Transform Spectral Imager (HTSI) uses one Offner relay with a curved grating to disperse and reimage input light onto a DMD and a second Offner relay to de-disperse and reimage the selected components onto the detector array. Hampered by low contrast, low efficiency, sizable space requirements, and high fabrication costs, the HTSI system is optimized for telescopic point imaging of very small objects at a large distance. However, this system would provide very poor area imaging performance and would not be easily adapted for imaging extended objects or nearby objects, being prone to high levels of spectral crosstalk that would prevent satisfactory multicolor imaging.
While there have been a number of different design approaches adopted for programmable spectral imaging, there are significant drawbacks with each approach. For example, any one tunable filter component, such as the LCD and AO tunable filter devices described above, is designed to operate over a relatively narrow spectral range. These devices are temperature-sensitive, polarization sensitive, and provide generally poor contrast. The bandwidth of any one filter is generally fixed by filter design parameters; the tuning operation merely shifts the same transmission characteristics up or down the wavelength scale, without control of amplitude, bandwidth, or filter shape.
Referring to FIG. 1, there is shown a block diagram of the principal subsystems of a spectral imaging system 10 and the type of data that can be obtained. A light source 12 illuminates an object 14. A linear spectral imager 16 obtains spectral data from object 14, one line 18 at a time, as composite data of substantially all wavelengths over a range or as data of one or more selected wavelengths. In a scanning sequence, successive lines 18 can be obtained in order to form a two-dimensional image. Spectral imager 16 can optionally obtain complete spectral data for a single point location 20.
Spectral imaging system 10 can employ any of a number of types of imaging sensor, including both area and linear sensors. Depending on the design of spectral imager 16, spectral imaging system 10 may be a hyperspectral imaging system or a multi-spectral imaging system. In a typical hyperspectral imaging system, spectral imager 16 could use a grating or other dispersive component for dispersing the various spectral components of object 14 (FIG. 1) onto an area sensor. In a typical multispectral system, on the other hand, spectral imager 16 could contain a series of thin-film filters, for specific fixed wavelengths, directing light to parallel arrays of linear sensors. Multi-spectral imaging systems can provide high-resolution spectral images of large, extended objects or areas with excellent image quality, short detector integration times, and generally more manageable image data sets than hyperspectral imaging systems provide.
One class of spectral imaging system employs a spatial light modulator as a type of programmable spectral switch for directing selected bands of incident light obtained from an object field, in sequence, to a sensor. While this approach has been demonstrated successfully for point-imaging and for sensing apparatus that utilize a single detector element, however, it can be appreciated that there would be additional advantages to an imaging system of this type in which the programmable spectral switch provides a programmable equivalent to a color filter wheel. For such a system, it would be particularly advantageous to use a spatial light modulator that is highly efficient, provides high contrast, and operates at high switching speeds. It would also be advantageous to provide a type of programmable filter that would not only allow tuning of frequency such as can be obtained using some LCD tuned filters, AO filters, and Fabry-Perot filter components, but also allow some measure of control of key characteristics such as bandwidth and filter shape as well.
One particularly advantaged spatial light modulator is an electromechanical conformal grating device consisting of ribbon elements suspended above a substrate by a periodic sequence of intermediate supports, as disclosed by Kowarz in U.S. Pat. No. 6,307,663, entitled “Spatial Light Modulator With Conformal Grating Device” issued Oct. 23, 2001. The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of the '663 disclosure has more recently become known as the conformal GEMS device, or simply as the GEMS device, with GEMS standing for Grating ElectroMechanical System. The GEMS device possesses a number of attractive features. It provides high-speed digital light modulation with high contrast, high efficiency, and a relatively large active region. Significantly, the GEMS device is designed for on-axis illumination, unlike other types of high-speed electromechanical light modulators, such as the Digital Micromirror Device (DMD), that requires off-axis illumination angles. As a further advantage, the GEMS device can be fabricated as a linear device with a thin active area, able to modulate a thin line of an image at a time, or, alternately, can be fabricated with a relatively wide active area in order to modulate a wider segment of an image at one time.
The ability to analyze the spectral components of light from an object or device under test has great value in a number of industrial and product testing and inspection, medical diagnostic, and sensing applications. Thus, it can be seen that there is a need for a spectral imaging system that can be programmed to provide suitable frequency, bandwidth and filter shape characteristics for obtaining spectral data.